Thermoelectric conversion material, thermoelectric conversion device and manufacturing method thereof

ABSTRACT

A thermoelectric conversion material and a thermoelectric conversion device having a novel structure of an increased figure of merit are provided by forming nano-wires of thermoelectric material in a smaller cross-sectional size. The thermoelectric conversion material comprises nano-wires obtained by introducing a thermoelectric material (semiconductor material) into columnar pores of a porous body. The porous body is formed by providing a structure in which columns of a column-forming material containing a first component (for example, aluminum) are distributed in a matrix containing a second component (for example, silicon or germanium or a mixture of them) being eutectic with the first component, and then removing the column-forming material from the structure. The average diameter of the nano-wires of the thermoelectric material is 0.5 nm or more and less than 15 nm, and the spacing of the nano-wires is 5 nm or more and less than 20 nm.

TECHNICAL FIELD

The present invention relates to a thermoelectric conversion materialhaving a novel structure and a manufacturing method thereof. Moreparticularly it relates to a thermoelectric conversion material of anovel structure that has a high thermoelectric figure of merit in athermoelectric conversion device that converts heat to electricity orconverts electricity to heat, and also to a manufacturing methodthereof.

BACKGROUND ART

It is well known that if a thermoelectric conversion material such asbismuth (Bi), bismuth telluride (BiTe) or silicon-germanium (SiGe), hasa low dimensional structure such as a superlattice structure ornano-wire structure (quantum wire structure), it will have a largerthermoelectric figure of merit Z than in a bulk form (Hicks, L. D.,Dresselhaus, M. S., Phys. Rev. B., Vol. 47, 12727 (1993)). One mainreason of this is that low dimensional structure of the materialprovides a quantum effect and increases interface, which leads to amodified density of state and a modified phonon scattering withoutsubstantial change in resistivity, resulting in higher Seebeckcoefficient α and lower thermal conductivity than in a bulk form. Inparticular, a thermoelectric material in a nano-wire form has a largelymodified density of state (i.e., increase in density of state at bandedge) due to the quantum effect, and hence can have a largerthermoelectric figure of merit than in a two-dimension structure such asa super-lattice structure.

The figure of merit Z that is commonly used as an index of athermoelectric material is defined as follows:Z=α ²/χρ  (1)where α represents a Seebeck coefficient; χ represents a thermalconductivity; ρ represents a resitivity. As seen from the equation,increase in the Seebeck coefficient α or decrease in the thermalconductivity χ leads to increase in the figure of merit Z.

Accordingly, for increasing Z value, attempts have been made to producevarious thermoelectric materials (semiconductor materials) in anano-wire form (quantum wire form). For example, an attempt has beenmade to produce nano-wire of a thermoelectric material such as BiTe,BiSb or Bi by filling pores in a porous oxide film formed by anodizationof aluminum (anodized alumina) with such a material (BiTe, BiSb or Bi)(Amy L. Prieto, Melissa S. Sander, Marisol S. Martin-Gonzalez, RonaldGronsky, Timothy Sands, and Angelica M. Stacy “J. Am. Chem. Soc.” Vol.123, 7160-7161 (2001)).

Here, an anodization of aluminum will be briefly described. In ananodization process of aluminum, anodizing aluminum plate or aluminumfilm formed on a substrate in an electrolyte acid produces porous oxidefilm (anodized alumina) (R C. Furneaux, W. R. Rigby & A. P. Davidson“Nature” Vol. 337, P147 (1989)). This porous oxide film is characterizedby the geometrical feature that fine cylindrical pores of diameters ofseveral nanometers to several hundred nanometers (nano-holes) arearranged in parallel at spacing of several tens nanometers to severalhundred nanometers (cell sizes). Cylindrical pores those arranged atspacing of several tens nanometer or more have a high aspect ratio andrelatively uniform cross sectional diameter. The diameter and spacing ofthe pores can be controlled to a certain degree by properly selectingthe acid species and the voltage for anodization.

Thus, such anodized oxide film of aluminum can be used as a mold toproduce a thermoelectric material in a nano-wire form, which willincrease figure of merit Z.

However, in the anodization process of aluminum, when the anodizationvoltage is adjusted to form pores at spacing of 10 nm or less so as toobtain nano-wires at a high density, it is difficult to form adjacentpores separated from each other by anodized alumina wall 93 on asubstrate 94 as shown in FIG. 9, that is, pores tend to communicate eachother. In this situation, the anodized alumina 93 contains a more numberof non-isolated pores 92 than isolated pores 91. Thus, it is difficultto produce pores separated by alumina walls with spacing of 10 nm orless, and a large area is required to produce a large number ofnano-wires.

According to a theoretical calculation, the smaller the size (diameter)of nano-wire is, the greater the figure of merit Z becomes. However, theanodization of aluminum can only produce pores or nanowires of a size(diameter) of about 7 to 9 nm, and it is difficult to form pores of across-sectional size (or diameter) less than 7 nm. In other words, it isdifficult to increase figure of merit by producing nano-wires of across-sectional size (diameters) of 7 nm or less.

Accordingly, it is an object of the present invention to provide athermoelectric conversion material and a thermoelectric conversiondevice of a novel structure by forming nano-wires of a thermoelectricmaterial to have a narrower size at a higher density than thoseconventionally fabricated, so as to increase the figure of merit Z.

It is also an object of the present invention to provide a manufacturingmethod to easily produce such a thermoelectric conversion material of anovel structure.

DISCLOSURE OF THE INVENTION

A first aspect of the present invention is a thermoelectric conversionmaterial having a multi-column structure which comprises a porous bodyhaving columnar pores and a semiconductor material that can performthermoelectric conversion introduced into the pores of the porous body,characterized in that the porous body is formed by removing the materialforming columns from a structure in which a plurality of columns of acolumn-forming material containing a first component are distributed ina matrix containing a second component being eutectic with the firstcomponent.

A second aspect of the present invention is a thermoelectric conversionmaterial having a multi-column structure, characterized in that thecolumn structure is obtained by the steps of: providing a porous bodyhaving a plurality of columnar pores, which porous body is formed byremoving the material forming the columns from a structure in which aplurality of columns of a column-forming material containing a firstcomponent are distributed in a matrix containing a second component thatis eutectic with the first component, introducing into the pores asemiconductor material that can perform thermoelectric conversion; andthen removing the porous body.

Preferably, the porous body is a thin film.

According to the present invention, it is possible to obtain nano-wiresof a thermoelectric material of which cross-sectional size and densitycannot be achieved by the conventional anodization of aluminum.

According to the present invention, it is possible to obtain a structurecomprising a plurality of columns and a matrix surrounding the columns,wherein the columns have a Seebeck coefficient at a room temperaturelarger than when the material forming columns is in a bulk solid. Thepresent invention also provides a thermoelectricity conversion devicecomprising on a substrate, a structure which comprises columns of amaterial and a matrix surrounding the columns, wherein the columns havea Seebeck coefficient larger than that of the material in a bulk solidat room temperature, and the columns are electrically connected toelectrodes; and the device generates current flow in response to thermalchange of outside.

In the present invention, the porous body may be subject to a chemicaltreatment before the semiconductor material is introduced into thepores. The chemical treatment is desirably an oxidation treatment. Sucha chemical treatment (oxidation treatment) of the porous body allows theporous body to be chemically stabilized. In some cases, the chemicaltreatment (oxidation treatment) can decrease the thermal conductivity ofthe porous body to the level lower than that of anodized alumina,thereby increase the efficiency of the resulting thermoelectricconversion device.

In the present invention, preferably the column-forming material isaluminum; the matrix is silicon; and the structure has 20 atomic %(inclusive) to 70 atomic % (inclusive) of silicon. Alternatively,preferably the column-forming material is aluminum; the matrix isgermanium; and the structure has 20 atomic % (inclusive) to 70 atomic %(inclusive) of germanium.

In the present invention, the main component of the porous body issilicon or germanium or complex thereof except for oxygen. Such acomposition allows formation of nano-wires of a thermoelectric material,of which density and cross-sectional size cannot be achieved by theanodization of aluminum.

The cross-sectional size of a column in the column-containing structureis desirably between 0.5 nm (inclusive) and 15 nm (inclusive). Such across-sectional pore size can provide a higher thermoelectric figure ofmerit.

The spacing of columns in the column-containing structure is desirablybetween 5 nm (inclusive) and 20 nm (inclusive). Such spacing can providehigher density of nano-wires of thermoelectric material.

Part of the column-forming material is desirably crystalline material,and the matrix is desirably of amorphous material.

According to one aspect of the present invention, there is provided amanufacturing method of a thermoelectric conversion material of thepresent invention that comprises the steps of: providing a structure inwhich columns of a column-forming material containing a first componentare distributed in a matrix containing a second component that iseutectic with the first component: removing the column-forming materialfrom the structure to obtain a porous body; and introducing asemiconductor material into the pores of the porous body.

The method may have a further step of removing the matrix after theintroduction step. The method may also have a step of chemicallytreating the porous body after the removal step. The chemical treatmentis desirably an oxidation treatment. The removal step is desirablyetching. The introduction step is preferably electrodeposition.

The semiconductor material is typically, but not limited to, an alloycrystal composed of Bi, Sb, Te, and/or Se, such as BiSb or BiTe, and itmay also be other various materials used as a thermoelectric conversionmaterial in a bulk form, such as Si, SiGe, etc.

Investigating microstructures containing aluminum, the inventors of thepresent invention found that in preparation of aluminum film on asubstrate by using a film deposition method in a non-equilibrium statesuch as sputtering, when silicon and/or germanium are added in apredetermined ratio to aluminum, multiple aluminum columns are formed insilicon or germanium or a mixture thereof in a self-organizing manner.The inventers also found that when the film containing columnar aluminumis immersed in a solution that dissolves aluminum but not silicon orgermanium or a mixture thereof, a porous body can be produced of whichfine cross-sectional size and high pore density cannot be achieved byanodization of aluminum.

The inventers found that oxidation treatment of the produced porous bodycan change the material constituting the porous body to an oxidematerial.

The inventors carried out an intensive study on the basis of the abovefindings to complete the present invention.

It is essential to use an aluminum-silicon (germanium) film in which theamount ratio of silicon (or germanium) to the total of aluminum andsilicon (or germanium) is between 20 and 70 atomic %, because only insuch a range, a nano-structure having multiple columns of aluminum canbe formed. In other words, if the content of silicon (or germanium) isless than 20 atomic % of the total amount of aluminum and silicon (orgermanium), the aluminum columns become 15 nm or more in cross-sectionalsize, while if the ratio of the amount of silicon (or germanium) to thesum amount of aluminum and silicon (or germanium) is more than 70 atomic%, columnar structure of aluminum cannot be identified by typicalscanning electron microscopes.

Introducing a semiconductor material by electrodeposition into such aporous body composed of silicon (or silicon oxide) or germanium (orgermanium oxide) can produces nano-wires with a small cross-sectionalsize (for example, not less than 0.5 nm and less than 15 nm) at a highdensity (for example, spacing not less than 5 nm and less than 20 nm).Note that the silicon (or silicon oxide) portion or the germanium (orgermanium oxide) portion constituting the porous body may be removedafter the formation of the nano-wires.

The structure from which the porous body is obtained (mother structure)will be described.

The mother structure used in the present invention comprises a firstcomponent and second component, in which columns (column-formingmaterial) containing the first component are surrounded by a matrixcontaining the second component. In this constitution, the motherstructure desirably contains the second component in a content not lessthan 20 atomic % and less than 70 atomic % of the total of the firstcomponent and second component.

The content, which is here referred to the ratio of the amount of thesecond component to the sum of the first component and second component,is preferably between 25 atomic % (inclusive) and 65 atomic %(inclusive), and more preferably between 30 atomic % (inclusive) and 60atomic % (inclusive).

Note that the term “column-forming material” or “columns” refers tothose forming substantially columnar forms, and may further contain thesecond component, and the matrix may further contain the firstcomponent. The column-forming material and the matrix surrounding it maycontain small amounts of oxygen, argon, nitrogen and/or hydrogen.

The ratio can be determined quantitatively by, for example, inductivelycoupled plasma emission spectroscopic analysis. The values of the ratiodescribed above are in atomic %. The range between 20 atomic %(inclusive) to 70 atomic % (inclusive) corresponds to the range between20.65 wt % (inclusive) and 70.84 wt % (inclusive), with the atomicweight of Al 26.982 and the atomic weight of Si 28.086.

The first and second components are preferably a combination ofmaterials having an eutectic point in a phase diagram of them (so calledeutectic materials). Specifically the eutectic point is 300° C. orhigher, and preferably 400° C. or higher. A preferable combination ofthe first and the second components may be a combination of Al (as thefirst component) and Si (as the second component), a combination of Al(as the first component) and Ge (as the second component), or acombination of Al (as the first component) and Si_(x)Ge_(1-x) (0<x<1)(as the second component).

The cross-section of the column-forming material is circular or oval. Inthe structure, the columns are distributed in a matrix containing thesecond component. The cross-sectional sizes of the columns (for circularcross sections, diameters) can be controlled as a function of thecomposition of the structure (or the content of the second component)and the average size of them is between 0.5 nm (inclusive) and 50 nm(inclusive), and preferably between 0.5 nm (inclusive) and 20 nm(inclusive), and more preferably between 0.5 nm (inclusive) and 10 nm(inclusive). In the case of an oval or the like, the major axis ispreferably between such ranges. Here, the “average size” means thatwhich is derived, directly from or through computer image processing of,actual picture images of columnar portions observed by SEM imaging(about a range of 100 nm by 70 nm). The lower limit of the averagecross-sectional size for practical use is 1 nm or larger, or several nmor larger.

The center-to-center distance of the columns, 2R is between 2 nm(inclusive) and 30 nm (inclusive), and preferably between 5 nm(inclusive) and 20 nm (inclusive), and more preferably between 5 nm(inclusive) and 15 nm (inclusive). Note that the lower limit of thecenter-to-center distance 2R should be determined at least such that thecolumns have adequate spacing so that they do not contact with eachother.

The structure is preferably one in a film form, and in this case, thecolumns are distributed in a matrix containing the second componentwhere the columns are substantially perpendicular to the film plane.There is no specific limitation on the thickness of the film, and thethickness may be between 1 nm and 100 μm. Considering processing time,etc., the practical thickness is between 1 nm and 50 μm. Preferably, afilm of 300 nm or thicker still has columnar-containing structure.

The structure is preferably one in a film form, and may be formed on asubstrate. The substrate may be, but not limited to, an insulatorsubstrate such as quartz glass, a semiconductor substrate such as asilicon substrate, gallium arsenide substrate or indium phosphidesubstrate, or, if the structure can be formed on a metal substrate or asubstrate (a support matrix), a flexible substrate (of polyimide, forexample).

The structure can be fabricated using a film deposition method conductedin a non-equilibrium condition. Such a film deposition method ispreferably sputtering, but any of other film deposition methods forforming a material in any non-equilibrium condition can be usedincluding resistance heating evaporation, electron-beam evaporation (EBevaporation) or ion plating. In the case of sputtering, it may bemagnetron sputtering, RF sputtering, ECR sputtering or DC sputtering. Inthe case of sputtering, the film deposition is performed typically in anargon atmosphere with a pressure in a reactor on the order of 0.01 Pa to1 Pa. In the sputtering, two individual material targets, or a firstmaterial target and a second material targets may be used, oralternatively a material target can be used which contains the firstmaterial and second material that are sintered with a predeterminedratio.

The structure formed on a substrate is formed in the temperature ofsubstrate between 20° C. (inclusive) and 300° C. (inclusive), andpreferably between 20° C. (inclusive) and 200° C. (inclusive).

Removing the column-forming material from the structure (by wet etchingor dry etching) produces a porous body that contains multiple columnarpores. The etching only has to selectively remove the column-formingmaterial, and the etchant is preferably an acid such as phosphoric acid,sulfuric acid, hydrochloric acid or nitric acid. The pores in the porousbody produced by the removal are preferably isolated from each other ornot connected to each other.

The method for fabricating the porous body from the structure desirablyhas: a step of providing the structure containing a first component anda second component in which the column-forming material containing thefirst component are surrounded by a matrix containing the secondcomponent, and the structure contains the second component at such aratio that the amount of the second component to the sum of the firstcomponent and second component being between 20 atomic % (inclusive) and70 atomic % (inclusive); and a step of removing the column-formingmaterial from the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a thermoelectric conversion materialaccording to the present invention;

FIG. 2 is a process flow chart of a manufacturing method of athermoelectric conversion material of the present invention;

FIG. 3 is a process flow chart of another manufacturing method of athermoelectric conversion material of the present invention;

FIG. 4 illustrates a manufacturing method of a thermoelectric conversionmaterial according to the present invention;

FIG. 5 is a schematic diagram of a thermal conversion material ofExample 1;

FIG. 6 is a schematic diagram of a thermal conversion material ofExample 2;

FIG. 7 is a schematic diagram of a thermal conversion material ofExample 3;

FIG. 8 is a schematic diagram of an exemplary thermoelectric conversiondevice employing a thermoelectric conversion material of the embodimentsand examples of the present invention; and

FIG. 9 is an exemplary cross-section view of anodized alumina in theprior art with the spacing of pores being 10 nm or less.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of thermoelectric conversion materials and manufacturingmethods thereof according to the invention will be described withreference to the accompanied drawings.

[Structure of Thermoelectric Conversion Material]

FIG. 1 is a schematic diagram of an exemplary thermoelectric conversionmaterial according to the embodiment. In this example, a thermoelectricconversion material is shown in which quantum wires (hereinafter,referred to as nano-wires) of a thermoelectric material havingcross-sectional sizes of several nm (nanometers) to several tens nm areformed in pores on a substrate. In FIG. 1, the reference numeral 11refers to a film form of thermoelectric conversion material; thereference numeral 12 refers to thermoelectric material formed asnano-wires constituting the thermoelectric conversion material 11(hereinafter, referred to as nano-wire(s) if necessary); the referencenumeral 13 refers to a substrate; and the reference numeral 14 refers toa porous body.

The nano-wires 12 are provided in the porous body 14. As shown in FIG.1, the nano-wires 12 are separated from each other by the porous body14, and are provided perpendicularly or substantially perpendicularly tothe substrate 13. The shape of the nano-wires 12 is columnar, as shownin FIG. 1. The diameter of the nano-wires 12 (the average diameter ofnano-wires 12 viewed from the film surface) is between 0.5 nm(inclusive) and 15 nm (not inclusive), and the spacing of the nano-wires12 (the average center-to-center distance of the nano-wires viewed fromthe film surface) is between 5 nm (inclusive) and 20 nm (not inclusive).

The porous body 14 constituting the thermoelectric conversion material11 is formed by removing the column-forming material containing a firstcomponent distributed in a matrix containing a second component that iseutectic with the first component. The column-forming materialcontaining the first component consists of, for example, a material thatcontains aluminum as the main component. The matrix containing thesecond component being eutectic with the first component, for example,germanium or silicon or a mixture of germanium and silicon.

The material of the porous body 14 preferably contains silicon (orsilicon oxide) or germanium (or germanium oxide) as the main component.Alternatively, it may contain a mixture of silicon and germanium (oroxide of the mixture) as the main component. The material of the porousbody 14 desirably contains silicon or germanium (or oxide thereof) asthe main component, and may contain several to several tens atomic % ofaluminum (Al), argon (Ar), nitrogen (N) and/or hydrogen (H).

While the material of the porous body 14 is preferably amorphous, it maycontains crystalline material.

The material constituting the nano-wires is typically an alloy crystalconsisting of Bi, Sb, Te and/or Se such as Bi, BiSb or BiTe, but notlimited thereto. Rather, it may be any of various materials that areconventionally used as a thermoelectric conversion material in a bulkform.

In FIG. 1, the thermoelectric material 11 is not limited to the abovedescribed configuration and may have a configuration formed afterremoving the porous body 14 separating the thermoelectric material 12.

[Manufacturing Method of Thermoelectric Conversion Material]

Manufacturing methods of the thermoelectric conversion materialaccording to the present invention is described below.

FIG. 2 is a process flow chart of an embodiment of the manufacturingmethod of the thermoelectric conversion material. The manufacturingmethod shown in FIG. 2 has steps (a) to (c) as follows:

(a): a step of providing a structure in which columns of a materialcontaining a first component are distributed in a matrix containing asecond component that is eutectic with the first component; then

(b): a step of removing the column-forming material from the structureto obtain a porous body; and then

(c): a step of introducing a semiconductor material into the pores ofthe porous body.

FIG. 3 is a process flow chart of another embodiment of themanufacturing method of the thermoelectric conversion material:

(a): a step of providing a structure in which columns of a materialcontaining a first component are distributed in a matrix containing asecond component that is eutectic with the first component; then

(b): a step of removing the column-forming material from the structureto obtain a porous body;

(c): a step of chemically treating (for example, oxidizing) the porousbody; and then

(d): a step of introducing a semiconductor material into the pores ofthe porous body.

A manufacturing method of the thermoelectric conversion material will bedescribed more specifically with reference to the drawings.

FIG. 4 illustrates an exemplary manufacturing process of athermoelectric conversion material of this embodiment. Steps (a) to (c)are described one by one.

Step (a): A structure in which columns of a material containing a firstcomponent 41 are distributed in a matrix containing a second component44 that can form eutectic with the first component 41 is provided.

Here, for example, aluminum (first component 41) and silicon (orgermanium) (second component 44) are provided to form columns in amatrix. Then, a structured film of a mixture 43 (aluminum-siliconmixture film or aluminum-germanium mixture film) is formed on asubstrate 42 by using a method such as sputtering that can produce afilm in a non-equilibrium state.

When an aluminum-silicon mixture film (or aluminum-germanium mixturefilm) 43 is formed by using such a method, the aluminum and silicon (orgermanium) form an eutectic structure in a meta-stable state in whichaluminum component separates and forms a nano-structure containingmulti-columns of several nm in the matrix in a self-organizing manner.Such aluminum columns are in a shape of circular cylinder of a diameterbetween 0.5 nm (inclusive) and 15 nm (not inclusive) and the columnspacing is between 5 nm (inclusive) and 20 nm (not inclusive).

In the aluminum-silicon mixture film (or aluminum-germanium mixturefilm) 43, silicon (or germanium) is contained in a range of 20-70 atomic% of the total content of aluminum and silicon (or germanium) in thefilm and preferably between 25 and 65 atomic %, and more preferablybetween 30 and 60 atomic %. Silicon content in such a range allowsformation of the aluminum-silicon mixture film (or aluminum-germaniummixture film) 43 in which columnar aluminum is distributed in a matrixof silicon (or germanium).

Here, the ratio of silicon to aluminum is represented by “atomic %”(atom % or at %), that is, in a ratio of the number of silicon (orgermanium) atoms to that of aluminum. Such atomic % is obtained byquantitative analysis of silicon (or germanium) and aluminum in thealuminum-silicon mixture film (or aluminum-germanium mixture film) 43,for example, by using inductively coupled plasma emission spectroscopicanalysis (ICP).

Step (b): Then the column-forming material is removed.

Here, for example, aluminum, the column-forming material in thealuminum-silicon mixture film (or aluminum-germanium mixture film) 43 isetched away with phosphoric acid to form pores 46 in the matrix (here,silicon or germanium). This produces a porous body 45 on the substrate42.

The pores 46 in the porous body 45 have spacing from 5 nm (inclusive) to20 nm (not inclusive) and its cross-sectional size is from 0.5 nm(inclusive) to 15 nm (not inclusive).

The etching solution can be a solution of an acid such as phosphoricacid, sulfuric acid, hydrochloric acid or chromic acid that dissolvesaluminum but hardly dissolves silicon or germanium. However, it may bean alkaline solution such as aqueous sodium hydroxide as long as it doesnot have adverse-effect on the pore formation by etching, and thusshould not be limited to specific types of acid or alkali. A mixture ofacid solutions or alkaline solutions may also be used. The etchingconditions such as solution temperature, concentration and time can beselected in a suitable manner depending on the porous body to beproduced.

Step (c): A thermoelectric material (semiconductor material) 47 isintroduced into the pores of the porous body produced by the removalstep. Thus, the thermoelectric material 47 becomes nano-wires.

In this step, the porous body is filled with the thermoelectric material47. For example, Bi or BiTe is filled in to pores by electrodeposition.The thermoelectric material 47 is typically BiSb or BiTe having a highthermoelectric figure of merit inherently, but not limited to them, andother various material may be used that are used as thermoelectricconversion materials in a bulk form.

The method for filling the pores with the material is preferablyelectrodeposition, and may also be a catalytic reaction method or VLS.

[Configuration of Thermoelectric Conversion Device]

FIG. 8 is a schematic diagram of an exemplary thermoelectric conversiondevice of this embodiment. Here, the “thermoelectric conversion device”means either a thermoelectric generating device that converts heat toelectricity, or a thermoelectric cooling device that provides a coolingeffect by the current flowing therethrough. FIG. 8 shows an example ofsuch a thermoelectric generating device. The thermoelectric generatingdevice of the present invention comprises a section of p-typethermoelectric conversion material 103 and a section of n-typethermoelectric conversion material 105. Either thermoelectric conversionmaterial section comprises a plurality of nano-wires (102 or 104) and aporous body 101. FIG. 8 shows only a pair of p-type thermoelectricconversion material section 103 and n-type thermoelectric conversionmaterial section 105. However, a typical configuration of the device hasa plurality of the pairs arranged in series.

Here, a higher temperature electrode 108 and lower temperatureelectrodes 107 and 106 are not supported on support plates. Howevertypical electrodes are desirably supported on a support plate.

EXAMPLES

The present invention will be described specifically with examples.

Example 1

In this example, a thermoelectric conversion material was produced inwhich the porous body having the columnar pores was amorphous silicon,and the semiconductor filled into the pores was BiTe.

First, an aluminum-silicon mixture film of about 200 nm thick containing37 atomic % of silicon to the total of aluminum and silicon was formedby magnetron sputtering on a silicon substrate on which 20 nm oftungsten was deposited as an electrode for electrodeposition of BiTe(thermoelectric material). As a target, a six 15-mm square silicon chipsare placed on a circular aluminum target of 4 inches in diameter (101.6mm). Sputtering conditions employed were such that supply was used withan Ar flow of 50 sccm, a discharging pressure of 0.7 Pa and input powerof 1 kW. The substrate temperature was room temperature (25° C.).

The aluminum-silicon mixture film thus obtained was observed by FE-SEM(Field Emission-Scanning Electron Microscope). When the surface wasviewed from above at an angle, it was found that round columns ofaluminum surrounded by the silicon matrix were arrangedtwo-dimensionally as shown in (a) of FIG. 4. The diameter of thecolumn-forming material of aluminum was 5 nm, and the average spacing(center-to-center distance) of them was 8 nm. FE-SEM observation of thecross-section shows that the columns of aluminum were isolated from eachother.

Then, the aluminum-silicon mixture film thus fabricated was immerses in98% concentrated sulfuric acid for 24 hours to selectively etch awayonly the column-forming material of aluminum so that pores were formed.As a result, a porous body was produced that consists of a materialcontaining silicon as the main component except for oxygen. The surfaceof the pores was oxidized.

The porous body consisting of matrix containing silicon as the maincomponent (the aluminum-silicon mixture film that had been subjected tothe etching with concentrated sulfuric acid) was observed by FE-SEM. Thesurface viewed from above at an angle had pores surrounded by thesilicon matrix, arranged two-dimensionally as shown in (b) of FIG. 4.The diameter of the pores was 5 nm, and the average spacing of them was8 nm.

Then, BiTe (semiconductor material) was filled into the pores of theporous body containing silicon as the main component. Here, a solutionof 1 mol/l nitric acid dissolving Bi and Te therein was used forelectrodeposition of BiTe. The electrodeposition was performed in thesolution with a reference electrode of Ag/AgCl at −1.0 V. Then, BiTeprotruded from the pores were polished away.

The BiTe nano-wires thus fabricated in the porous body was observed withan FE-SEM to show that the substrate surface viewed from above at anangle had BiTe nano-wires 57 arranged two-dimensionally surrounded bythe porous body 54 consisting of silicon as the main component, in athermoelectric conversion material 53 formed on the substrate 52 shownin FIG. 5. Viewed from a section, the nano-wire 57 had a form of column.The average diameter of the nano-wires 57 was 5 nm, and the averagecenter-to-center distance of the adjacent nano-wires 57 was about 8 nm.

Example 2

In this example, a thermoelectric conversion material was produced inwhich the main component of the porous body having the columnar poreswas silicon oxide, and the semiconductor filled into the pores was BiTe.

First, an aluminum-silicon mixture film of about 200 nm thick containing37 atomic % of silicon to the total of aluminum and silicon was formedby magnetron sputtering on a silicon substrate on which 20 nm oftungsten was deposited as an electrode for electrodeposition of BiTe(thermoelectric material). As a target, a six 15-mm square silicon chipsare placed on a circular aluminum target of 4 inches in diameter (101.6mm). Sputtering conditions employed were such that supply was used withan Ar flow of 50 sccm, a discharging pressure of 0.7 Pa and input powerof 1 kW. The substrate temperature was room temperature (25° C.).

The aluminum-silicon mixture film thus obtained was observed with anFE-SEM (Field Emission-Scanning Electron Microscope) to find that thesubstrate surface viewed from above at an angle had a feature in whichround columns of aluminum surrounded by the silicon matrix were arrangedtwo-dimensionally as shown in (a) of FIG. 4. The diameter of thecolumn-forming material of aluminum was 5 nm, and the average spacing(center-to-center distance) of them was 8 nm. FE-SEM observation of thecross-section showed that the columns of aluminum were isolated fromeach other.

Then, the aluminum-silicon mixture film thus fabricated was immerses in5 wt % phosphoric acid for 7 hours to selectively etch away only thecolumn-forming material of aluminum so that pores were formed. At thepoint, the silicon matrix that had been surrounding the aluminum columnswas oxidized. As a result, a porous body was produced consisting of amaterial containing silicon oxide as the main component.

The porous body mainly consisting of silicon oxide was observed byFE-SEM. The surface viewed from above at an angle had pores surroundedby the silicon oxide matrix, arranged two-dimensionally as shown in (b)of FIG. 4. The diameter of the pores was 5 nm, and the average spacingof them was 8 nm. FE-SEM observation of the cross-section showed thatthe pores were isolated from each other by the matrix mainly consistingof silicon oxide.

Then, BiTe (semiconductor material) was filled into the pores of theporous body containing silicon oxide as the main component. Here, asolution of 1 mol/l nitric acid dissolving Bi and Te therein was usedfor electrodeposition of BiTe. The electrodeposition was performed inthe solution with a reference electrode of Ag/AgCl at −1.0 V. Then, BiTeprotruded from the pores were polished away.

The BiTe nano-wires thus fabricated in the porous body was observed byFE-SEM. When the surface of a thermoelectric conversion material 63formed on the substrate 62 was viewed from above at an angle, it wasshown that BiTe nano-wires 67 were arranged two-dimensionally surroundedby the porous body 64 consisting of silicon oxide as the main component,as shown in FIG. 6. From observation of a section of the substrate, thenano-wire 67 had a form of column. The average diameter of thenano-wires 67 was 4 nm, and the average center-to-center distance of theadjacent nano-wires 67 was about 8 nm.

Example 3

In this example, a thermoelectric conversion material was produced inwhich the material of the porous body having the columnar pores wasgermanium, and the semiconductor filled into the pores was BiSb.

First, an aluminum-germanium mixture film of about 200 nm that contained37 atomic % of germanium relative to the sum amount of aluminum andgermanium was formed by magnetron sputtering, on a silicon substrate onwhich tungsten of 20 nm thick had been deposited thereon as theelectrode for electrodeposition of BiSb (thermoelectric material). Atarget was used in which four 15-mm square germanium chips are placed ona circular aluminum target having a diameter of 4 inches (101.6 mm).Sputtering conditions were employed where RF power supply was used withan Ar flow: 12 sccm, a discharging pressure: 0.05 Pa and input power: 60W. The substrate temperature was room temperature (25° C.).

The aluminum-germanium mixture film thus obtained was observed with anFE-SEM, and it was then found that the substrate surface viewed fromupper side with slant had a feature in which substantially circularcolumn-forming material of aluminum were arranged two-dimensionallywhile surrounded by the germanium matrix as shown in (a) of FIG. 4. Thediameter of the columns of aluminum was 10 nm, and the averagecenter-to-center distance of them was 15 nm.

Then, the aluminum-germanium mixture film that contained 37 atomic % ofgermanium relative to the sum amount of aluminum and germanium wasimmerses in 98% concentrated sulfuric acid for 24 hours to selectivelyetch away only the column-forming material of aluminum so that poreswere formed. As the result, a porous body was produced containinggermanium as the main component.

The aluminum-germanium mixture film that had been subjected to theetching with concentrated sulfuric acid (the porous body consisting ofmaterial containing germanium as the main component) was observed byFE-SEM, and it was then found that the substrate surface viewed fromupper side with slant had a feature in which pores were arrangedtwo-dimensionally while surrounded by the germanium matrix as shown in(b) of FIG. 4. The diameter of the pores, 2r was 10 nm, and the averagespacing of them was 15 nm. Thus, the porous body containing germanium asthe main component was produced. The fabricated sample was subjected toX-ray diffraction analysis and found to be amorphous.

Then, semiconductor material was filled into the porous body thusproduced containing germanium as the main component. Here, BiSb wasfilled into the porous body to produce BiSb nano-wires in the porousbody. Here, electrodeposition of BiSb was employed with a solution ofdimethyl sulfoxide (DMSO) in which Bi(NO₃)₃.5H₂O and SbCl₃ weredissolved. The electrodeposition was performed in the solution with areference electrode of Ag/AgCl at −1.0 V. Then, the BiSb portionsprotruded from the pores were polished away.

The BiSb nano-wires thus fabricated formed in the porous body wasobserved by FE-SEM, and it was then found that the surface viewed fromupper side with slant had a feature in which BiSb nano-wires 77 werearranged two-dimensionally while surrounded by the porous body 74containing germanium as the main component, in a thermoelectricconversion material 73 formed on the substrate 72 shown in FIG. 7.Viewed from a section of the substrate, the nano-wire 77 had a form ofcolumn. The average diameter of the nano-wires 77 was 10 nm, and theaverage center-to-center distance of the adjacent nano-wires 77 wasabout 15 nm.

As described in the above examples, according to the present invention,when semiconductor material (thermoelectric material) is filled into aporous body which is formed by providing a structure in which columns ofa material containing a first component are distributed in a matrixcontaining a second component that can form eutectic with the firstcomponent, and then removing the column-forming material from thestructure, this allows the formation of nano-wires of thermoelectricmaterial with a diameter between 0.5 nm (inclusive) and 15 nm (notinclusive) and high density (the center-to-center distance of thenano-wires less than 20 nm).

The material constituting the porous body may be any of variousmaterials, such as silicon or germanium.

Example 4

In this example, a thermoelectric generating device was produced inwhich BiTe was employed as an n-type thermoelectric material and BiSbwas used as a p-type thermoelectric material.

First, an aluminum-silicon mixture film of about 2 μm that contained 50atomic % of silicon relative to the sum amount of aluminum and siliconwas formed by magnetron sputtering, on a silicon substrate havingsilicon oxide surface (a support plate) on which 20 nm of tungsten hadbeen deposited. Then, the aluminum-silicon mixture film that contained50 atomic % of silicon relative to the sum amount of aluminum andsilicon was immerses in 5 wt % phosphoric acid for 8 hours toselectively etching only the column-forming material of aluminum so thatpores were formed. As the result, a porous body was produced thatconsists of material containing silicon oxide as the main component.Then, BiTe (n-type thermoelectric material) was electrodeposited. Then,patterns of resist were formed by photolithography, and patterns of then-type thermoelectric conversion material were generated by dry etching.Using similar process steps, BiSb (p-type thermoelectric material) waselectrodeposited on the porous body and the silicon substrate withsilicon oxide (a support plate) with 20 nm of tungsten depositedthereon; patterns of resist were formed by photolithography; andpatterns of the p-type thermoelectric conversion material were generatedby dry etching. The silicon substrate with silicon oxide on which p-typethermoelectric material is formed, and the silicon substrate withsilicon oxide on which n-type thermoelectric material is formed areattached together to form a thermoelectric conversion device, as shownin FIG. 8.

A thermoelectric conversion material employing a thermoelectricconversion material obtained by the above embodiments and examples willbe described with reference to FIG. 8. A thermoelectric conversiondevice shown in FIG. 8, like known thermoelectric conversion devices inbulk form, is used for: a device such as a cooler or thermal controller,which performs both cooling and heating where current flowing through amaterial causes heat generation at one end of the material and heatabsorption at the other end because of Peltier effect; and a device suchas thermoelectric generator, which generate electromotive force(thermoelectromotive force) by providing a temperature difference acrossa material (this is the opposite effect to that of the above one).

In FIG. 8, the thermoelectric conversion device is embodied as an unitcomposing of multiple devices (π-type devices) connected in series witheach π-type device consisting of: a thermoelectric conversion materialsection 103 having nano-wires 102 of p-type semiconductor material(thermoelectric material) formed in a porous body 101 (hereinafter,referred to as “p-type material section” 103); and a thermoelectricconversion material section 105 having nano-wires 104 of n-typesemiconductor material (thermoelectric material) formed in a porous body101 (hereinafter, referred to as “n-type material section”) 105. In FIG.8, reference numeral 106 refers to an electrode provided on one end ofthe p-type material section 103 (hereinafter, referred to as lowertemperature-side); reference numeral 107 refers to an electrode providedon one end of the n-type material section 105 (hereinafter, referred toas lower temperature-side); and reference numeral 108 refers to anelectrode provided on the other ends of the material sections 103 and105.

In the case where the thermoelectric conversion device is applied to adevice that uses such an device as a thermoelectric generating device, aplurality of the ℏ-type devices of FIG. 8 are connected in series.Temperature difference between the upper electrode 108 and the lowerelectrodes 106 and 107 can cause the generation of electric power. Hereis illustrated the case where the upper electrode 108 is in a highertemperature while the lower electrodes is in a lower temperature,thereby generating electromotive force between the lower electrodes 106and 107 with the lower electrode 106 being positive and the lowerelectrode 107 being negative. The thermoelectric conversion device canalso be used as a cooling device, in which the electrode 106 isconnected to a negative terminal of a power supply and the electrode 107is connected to a positive terminal of the power supply, and currentflowing therethrough can cause heat absorption from the upper electrode108 in FIG. 8. Thus, cooling around the upper electrode 108 can beperformed. Such a thermoelectric conversion device can have a higherthermoelectric conversion figure of merit Z than conventionalthermoelectric conversion devices.

Note that the present invention is not limited the embodiments, examplesand applications illustrated above, but those skilled in the art canvary and modify them based on the description of the claims withoutdeparting from the gist of the present invention. Such variations andmodifications are also in the scope of the present invention.

As described in the above examples, according to the present invention,when semiconductor material (thermoelectric material) is filled into aporous body which is formed by providing a structure in which columns ofa material containing a first component are distributed in a matrixcontaining a second component that can form eutectic with the firstcomponent, and then removing the column-forming material from thestructure, this allows the formation of nano-wires of thermoelectricmaterial with a diameter between 0.5 nm (inclusive) and 15 nm (notinclusive) and high density (the spacing of the nano-wires less than 20nm). A thermoelectric conversion device employing such thermoelectricconversion device can also be provided. The present invention can alsoprovide a production method allowing easy production of thethermoelectric conversion device.

1. A thermoelectric conversion material having a multi-column structurecomprising a porous body having columnar pores and a semiconductormaterial that can perform thermoelectric conversion introduced into thepores of the porous body, characterized in that the porous body isformed by removing a column-forming material containing a firstcomponent from a structure in which a plurality of columns of thecolumn-forming material are distributed in a matrix containing a secondcomponent that is eutectic with the first component.
 2. A thermoelectricconversion material having a multi-column structure, characterized inthat the column structure is obtained by: providing a porous body havinga plurality of columnar pores which is formed by removing from astructure in which a plurality of columns of a column-forming materialcontaining a first component are distributed in a matrix containing asecond component that can form an eutectic with the first component,introducing into the pores a semiconductor material that can performthermoelectric conversion; and then removing the porous body.
 3. Thethermoelectric conversion material according to claim 1, wherein theporous body is in a thin film.
 4. The thermoelectric conversion materialaccording to claim 1, wherein the multi-column structure is obtained byfurther chemically treating the porous body and then introducing thesemiconductor material into the pores.
 5. The thermoelectric conversionmaterial according to claim 4, wherein the chemical treatment is anoxidation treatment.
 6. The thermoelectric conversion material accordingto claim 1, wherein the first component is aluminum; the secondcomponent is silicon; and the structure contains silicon at 20 atomic %or more and 70 atomic % or less.
 7. The thermoelectric conversionmaterial according to claim 1, wherein the first component is aluminum;the second component is germanium; and the structure contains germaniumat 20 atomic % or more and 70 atomic % or less.
 8. The thermoelectricconversion material according to claim 1, wherein a main component ofthe porous body other than oxygen component is silicon.
 9. Thethermoelectric conversion material according to claim 1, wherein a maincomponent of the porous body other than oxygen is germanium.
 10. Thethermoelectric conversion material according to claim 1, wherein theaverage diameter of columns in the structure is 0.5 nm or more and 15 nmor less.
 11. The thermoelectric conversion material according to claim1, wherein the average spacing of columns in the structure is 5 nm ormore and 20 nm or less.
 12. The thermoelectric conversion materialaccording to claim 1, wherein part of the column-forming material is acrystalline material, and the matrix is an amorphous material.
 13. Athermoelectric conversion device using a thermoelectric conversionmaterial according to claim
 1. 14. A manufacturing method of athermoelectric conversion material comprising the steps of: providing astructure in which a plurality of columns of a column-forming materialcontaining a first component are distributed in a matrix containing asecond component that is eutectic with the first component; removing thecolumn-forming material to form a porous body; and introducing asemiconductor material into pores of the porous body.
 15. Themanufacturing method according to claim 14, comprising a step ofchemically treating the porous body after the removal step.
 16. Themanufacturing method according to claim 14, wherein the chemicaltreatment is an oxidation treatment.
 17. The manufacturing method ofthermoelectric conversion material according to any one of claim 14 to16, wherein the introduction step of the semiconductor iselectrodeposition.
 18. A structure comprising a plurality of columns ofa column-forming material and a matrix surrounding the columns, whereinthe columns have a Seebeck coefficient at a room temperature larger thanthat of the material in bulk solid.
 19. The structure according to claim18 wherein the columns are placed on a substrate, and substantiallyperpendicular to a surface of the substrate.
 20. A thermoelectricityconversion device comprising on a substrate, a structure which comprisescolumns of a column-forming material and a matrix surrounding thecolumns, wherein the columns have a Seebeck coefficient larger than thatof the material in a bulk solid at room temperature, and the columns areelectrically connected to electrodes; and the device generates currentflow in response to thermal change of outside.